Still think the overall Mastery/Aegis Design is very flawed vs good teams that can notice crits etc.
Also necro's are still very strong vs Discs with there lack of Direct healing.

Think Disc in any Cleave will perform well in 5.3 without any serous changes to the game.

Disc/Enh/Warr (Turbo) is worth mentioning as a strong comp.

Shadow:

As for Shadow priests, I just see every priest getting trained to the ground by any Feral/Rogue/Warr etc with Swap nerf.
There needs to be some srs changes to Boomkins/Spriests/Locks/Ele to handle cleaves better.

Comp wise, think RPS is going to be the only viable 3s comp for Shadow.

Not sure if all this is right according to your opinions / current patch notes.

Not a priest, but as far as comps go: Shadowplay will still be decent imo with warlock/gateway buffs, Punchmonk/priest/rsham will be strong, Godcomp depending on what blizzard does to mages will still be strong, also with the new warrior buffs Warrior/priest/healer will be a viable comp.

Well if PvP Power Healing change for shadow will affect DP's healing (as it will affect Recuperate/Second Wind), then the void shift nerf won't be that big issue to keep yourself up while trained (3 orbs DP + VE + some healing like divine star/PoM). And since shadow will benefit three times more from PvP Power than now, for off-heals, you can spam flash heal into your partners until they are full/healer got out of cc. Only thing that seems to be really irritating is Mass Dispel nerf, Rets will still have their instant dispel on 30 sec cd, that doesn't cost 40k mana...

Well if PvP Power Healing change for shadow will affect DP's healing (as it will affect Recuperate/Second Wind), then the void shift nerf won't be that big issue to keep yourself up while trained (3 orbs DP + VE + some healing like divine star/PoM). And since shadow will benefit three times more from PvP Power than now, for off-heals, you can spam flash heal into your partners until they are full/healer got out of cc. Only thing that seems to be really irritating is Mass Dispel nerf, Rets will still have their instant dispel on 30 sec cd, that doesn't cost 40k mana...

Official Blizzard Quote:

Answer - (Systems Design Team):
In patch 5.3, all specs (even classes without healing specs) benefit from a PvP Power-to-healing conversion. This means that any heal should benefit from PvP Power; this includes percentage heals such as Second Wind and Recuperate. Previously things like Second Wind and Recuperate weren’t benefitting from PvP Power at all.